The present disclosure is related generally to passive power factor correction (PFC) circuits. More particularly, the present disclosure is directed to passive PFC circuits that provide a high power factor while utilizing a small number of passive components having reduced size.
Power factor is defined as the ratio of the real power to apparent power. It can be also defined as cosine of the phase angle between the current and voltage waveforms if both are pure sine waves. This occurs when the load is linear. The real power produces real work. The apparent power is the power that would be delivered to a pure resistive load, regardless of the current waveform. It is thus the total power supplied by an AC source to produce the required amount of the real power. The AC source is most often the power company supplying electricity through power lines but could also be the output of an electronic inverter, motor drive or other localized AC source. Due to energy stored in the load and returned to the source and/or due to a nonlinear load that distorts the wave shape of the current drawn from the source, the apparent power is greater than the real power. The power factor is 1.0 if both the input current and voltage are sinusoidal and in phase. This occurs when the load is or behaves like a resistor. It allows the power distribution system to operate at its maximum efficiency.
PFC may be needed in any line-powered device that uses AC/DC power conversion. These applications can range in scale from battery chargers for portable devices to big-screen TVs. Most commonly, the AC line voltage is rectified in a single phase bridge rectifier and filtered with a large electrolytic capacitor. These nonlinear and storage elements, also aided and abetted by the impedance of the power line itself, result in many problems such as reduction in the available power, increased losses and generation of serious harmonic distortions in the line current. The problems are magnified when operating a large numbers of these nonlinear loads due to the cumulative effect. The result is a poor power quality, wherein neutral currents can be large and rich in third harmonic currents.
Unless some correction circuit is used, the input rectifier with a capacitive filter circuit will draw pulsating currents from the AC source, resulting in poor power quality and high harmonic contents that adversely affect other users. The RMS value of the narrow pulses of the input current is higher than the corresponding sinusoidal current required to produce the same power. This situation has drawn the attention of regulatory bodies around the world. Governments are tightening regulations, setting new specifications for low harmonic currents and restricting the amount of harmonic currents that can be generated. This necessitates the need for PFC and harmonic reduction circuits.
PFC is required in various power systems supplied from line in order to comply with requirements of international standards such as EN61 000-3-2, Energy Star and 80 Plus. Without compliance to the appropriate standards, a product will have difficulties gaining acceptance in the marketplace. PFC is also necessary for energy saving. PFC usually reduces harmonics in the line current, increases the efficiency of the power systems and reduces customer's utility bill. In one estimate, the cost increase is directly proportional to the inverse of the power factor.
The methods to improve the power factor can be classified as active and passive methods. Active PFC circuits utilize feedback circuitry along with switching converters to synthesize input current waveforms consistent with high power factor. The advantages of the active PFC circuits include high power factor of at least 0.99, correction of both distortion and displacement, universal line voltage, regulated output voltage, small and light components, ability to absorb some line transients and design supported by vast array of integrated controllers. The disadvantages include complexity, output voltage that has to be greater than the peak of the input voltage, high cost especially for low power applications, no inrush current limiting, added conversion stage that decreases efficiency and increases EMI/RFI performance. The last shortcoming necessitates employment of a low pass filter at the input. The input ripple current is at the switching frequency of the active PFC circuit and must be filtered at the input. Unfiltered ripple will be conducted down the power line as EMI.
The passive PFC circuits incorporate passive components, typically capacitors and inductors. However, active components, such as synchronous rectifiers, emulating passive components can be used as well. The advantages of the conventional passive PFC circuits include simplicity, cost effectiveness especially at low power, high efficiency, reliability and ruggedness, no source of EMI/RFI, assistance with EMI/RFI filtering and capability of reaching unity power factor for linear loads. The disadvantages of the conventional passive PFC circuits include large and heavy line frequency components, inability to completely correct nonlinear loads, unregulated output voltage and component values dependent on load characteristics. The widely used valley-fill circuit is somewhat different. It employs rectifiers for diverting the current flow and capacitors for delivering energy to the load at low line voltage. However, the valley-fill circuit performs poorly when complementing the bridge rectifier and its storage capacitor.
Many applications do not require a power factor of at least 0.99 that the active PFC circuits commonly offer. The power factor of the passive PFC circuits can be tweaked in order to reduce size and cost of the components and yet meet performance requirements. For example, certain aircraft landing lights operating below 80 VA require leading power factor of 0.728 or lagging power factor of 0.613. Since an aircraft presents a very difficult EMI/RFI environment, the employment of the passive PFC is most advantageous. Moreover, many applications employ an input filter but no PFC due to disadvantages mentioned hereinabove. These applications could benefit greatly by replacing the input circuit, including low pass filter, with a passive PFC circuit without significantly raising the size and cost.
The present invention is intended to provide the passive PFC that incorporates many advantages of the PFC of both types while overcoming some disadvantages of the conventional passive PFC. Similarly to the active PFC circuits, the present PFC circuits are intended to expand the full-wave bridge rectifier and the following storage capacitor. The passive PFC technique according to the present disclosure specifically targets size of the inductor while avoiding any switching, and maintaining power factor that challenges active PFC circuits. Early prototypes reached power factor of 0.99 while driving a 100 W load.